1. Field of the Invention
Exemplary aspects of the present invention generally relate to a light projection unit, an image reading device using the same, and an image forming apparatus using the image reading device.
2. Description of the Background Art
Conventionally, there is known an image reading device that employs a light source that illuminates a surface of a document and an image pickup device, such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS), which receives reflected light from the document surface. An image on the document surface is read based on the reflected light from the document surface received by the image pickup device.
Typically, an image reading device that reads the document placed on a contact glass is equipped with a light projecting device. The light projecting device includes a carriage including a light source that moves along the contact glass while illuminating the document with light from the light source.
One example of a related-art light projecting device including the light source also includes a light guide made of translucent material disposed between the light source and the document. The light guide reflects and concentrates light radially emitted from the light source, which consists of a plurality of LEDs arrayed in a main scanning direction, onto a light emitting surface of the light guide. The advantage of such a configuration is that, even if the luminous intensity of the LEDs is relatively weak, light with high intensity can still be projected onto the document surface.
Ultimately, it is desirable to have as much of the light projected from the LEDs as possible to enter the light guide. Further, it is desirable to project as much of the light incident upon the light guide as possible onto an illumination target. Accomplishing these goals requires accurate positioning of the light source and the light guide both relative to each other and to the illumination target.
In the related-art light projecting device, the plurality of LEDs is arrayed on a substrate, hereinafter referred to as an LED array substrate. The light guide is fixed to the LED array substrate by double-sided tape or an adhesive agent. However, the light projecting device is not equipped with a mechanism to properly position the light guide relative to the LED array substrate. Thus, it is difficult to fix the light guide to the LED array substrate with precision.
In order to facilitate an understanding of the related art and of the novel features of the present invention, a description is now provided of an example of a related-art positioning mechanism for positioning the light guide, with reference to FIGS. 15 and 16. FIG. 15 is a schematic diagram illustrating one example of a related-art light projecting device. FIG. 16 is an explanatory diagram for explaining light paths of projected light.
As illustrated in FIG. 15, a holding member 605 is provided to hold the LED array substrate 691 and the light guide 603. As illustrated in FIGS. 15 and 16, the holding member 605 is longer than a document area F in the main scanning direction, in order to accommodate positioning holes 604a for positioning the light guide 603 formed at both ends of the holding member 605 outside the document area F.
The light guide 603 is also longer than the document area F in the main scanning direction. Positioning bosses 604 that engage the positioning holes 604a are formed at both ends of the light guide 603 outside the document area F. The light guide 603 is positioned by fitting the positioning bosses 604 into the positioning holes 604a. 
Although advantageous, there is a drawback to this configuration. As illustrated in FIG. 16, although only one end portion of the light guide 603 is illustrated, it can be seen that the end portions of the light guide 603 are separated apart from the end portions of the document area F. As a result, of all the light incident upon the light guide 603 from the LED 692 at the end portion of the LED array substrate 691 in the main scanning direction, that part of the incident light which is not headed to the document area F is uselessly projected outside the document area F as indicated by a broken-line circle X1, which is undesirable.
Consequently, of the light projected from the LED 693 at the end portion of the LED substrate 961 and incident upon the light guide 603 only the light heading toward the document area F shown by a solid-line circle Z1 is properly projected toward the document area F, while light intensity at the end portions of the document area F remains insufficient.
To correct this problem, it is preferable that the length of the light guide be substantially the same as that of the document area F. If the length of the light guide 603 in the main scanning direction is substantially the same as that of the document area F, the end portions of the document area F and both ends of the light guide 603 can be aligned as closely as possible. Such a configuration is illustrated in FIG. 17.
With this configuration, as illustrated in FIG. 17, of the light incident upon light guide 603 from the LED 692 at the end portions of the LED array substrate 691 in the main scanning direction, the incident light that is not headed to the document area F can still be directed toward the document area F as indicated by a broken-line circle X2. Accordingly, the light totally reflected from a side surface 603c of the light guide 603, indicated by the broken-line circle X2, can illuminate the document area F, in addition to the light directed toward the document area F indicated by the circle Z1, thereby reliably securing a sufficient amount of light at the end portions of the document area F.
When both end portions of the light guide 603 are aligned with the end portions of the document area F, the positioning bosses 604 need to be provided at places corresponding to the document area F. However, there is a drawback in this approach in that internal reflection does not occur in the positioning bosses 604, for the following reasons.
The light guide 603 is formed of resin, for example acryl, that is molded. The mold is mirror-finished so that the surface of the light guide 603 also has a mirror-like surface. However, a mirror finish cannot be given to the mold where the positioning bosses 604 are formed because these areas of the mold are depressed portions, that is, concavities. Consequently, a mirror finish cannot be given to the light guide 603. Furthermore, an incident angle of the incident light in the positioning bosses 604 is smaller than a critical angle. For these reasons, the light incident upon the positioning bosses 604 is not internally reflected but leaks out of the positioning bosses 604. As a result, the distribution of light over the document surface in the main scanning direction becomes significantly uneven.